This invention relates generally to locating underground objects, and more particularly the invention relates to radio transmission and reception in object locating systems and soil conductivity measurement systems without a direction ambiguity.
Buried pipes and conduits are used by a large number of utility companies including telephone, water, gas, CATV, and electric power. Frequently it becomes necessary to excavate in an area where one or more of these conduits have been placed. It also becomes necessary to repair broken or leaky conduits. Conversely, it is important to know the location of each of the conduits or pipes before any digging or excavation begins.
In order to facilitate the replacement and repair of buried utility system it is important to have a simple and quick method of locating the true position of the buried conduits. Most buried utility systems are constructed of materials through which an electric current can flow, and whenever a current flows a magnetic field is created. In cases where a pipe or conduit is made of a non-conductive or non-magnetic material, there is typically a wire or metal line placed in direct proximity therewith. AC currents can be induced in the conductive wire or pipe thus producing an AC magnetic field.
Soil conductivity measurements are often used for geophysical surveys. In this situation the conductive object would be something such as an ore body for which one may wish to create a map. Such systems can also detect less conductive bodies, such as tunnels, within more conductive materials.
There are a number of existing devices which are used to locate buried or otherwise concealed electrically conductive objects. The devices use an alternating current which is impressed on the concealed conductive object, such as a pipe or cable, by direct connection or by inductive coupling. The object can be located in a horizontal underground plane or in a lateral plane by use of a suitable horizontal axis pickup coil and amplifier with an appropriate indicating device such as a meter or audio transducer. When the receiver pickup coil is brought closer to the object being investigated, the AC signal level increases and the position nearest the object produces the strongest signal. Soil conductivity measurement systems typically use electrical contacts with the soil in place of the inductive pickup used in the utility locator case.
In such systems there can be an ambiguity in the received signal measurement. Since the signal is a sinusoid and no phase reference is brought from the transmitter to the receiver, there is no sign information in the received signal. The ambiguity arises because sin(t)=−sin(t+π).
In simple environments, assuming that the power is always flowing away from the transmitter, the received signal will give the correct results in identifying an underground object. In more complex physical arrangements, however, such as where a pipe makes a U-turn, the ambiguity can cause confusion.
Consider for example the arrangement in FIG. 1 where a pipe 10 has a reversal of direction at location 12 (a building for example) and extends at 10′ generally parallel to pipe 10. A signal detector might confuse signals from pipe 10 and pipe 10′ even though current is flowing in the two pipes in opposite directions. Thus, the operator might confuse pipe 10′ in locating pipe 10.
The present invention is directed to avoiding this ambiguity.